diff --git a/pymodbus/pdu/file_message.py b/pymodbus/pdu/file_message.py
index c4e3f7429..ea72c2afa 100644
--- a/pymodbus/pdu/file_message.py
+++ b/pymodbus/pdu/file_message.py
@@ -295,7 +295,7 @@ def calculateRtuFrameSize(cls, data: bytes) -> int:
         """Calculate the size of the message."""
         hi_byte = int(data[2])
         lo_byte = int(data[3])
-        return ((hi_byte << 16) + lo_byte) + 4
+        return ((hi_byte << 8) + lo_byte) + 4
 
     def __init__(
         self, values: list[int] | None = None, dev_id: int = 1, transaction_id: int = 0